The magnetic tape cassette consists of upper and lower half cases formed of plastics and fastened together, and a length of magnetic tape wound on a pair of reels in the space within the complete case. As the tape passes from one reel to the other via an external tape head, its both edges tend to run in direct contact with the inner surfaces of the upper and lower half cases, producing such a high frictional resistance that the tape running becomes unsteady and instable. In order to avoid this, it has been customary to insert pieces of lubricant sheet between the tape reels and the inner surfaces of the half cases. The lubricant sheet, in contact with the tape reels, helps the tape to run smooth and stably, and also serves to prevent troubles such as objectionable loosening of the tape and choking of its passageway.
Recently, it has been proposed to make the upper and lower half cases of magnetic tape cassettes from transparent plastics so that the interior of the cassettes can be seen from the outside to permit the user easily to see how much part of the tape has been wound on or unwound from which reel and watch the running condition of the tape. In addition, the transparency gives a look refined in design to the containers. In order that the cassettes may be built transparently to make the tape reels visible from the outside, it is necessary that the case itself is made transparent and additionally the lubricant sheet, to be interposed between the inner surfaces of the case and the reels, should be transparent, too.
The clear lubricant sheet for such application is required to meet some essential requirements including, of course, the ability of giving a low coefficient of friction on contact with the tape reels running within the cassette, and also freeness from electrostatic charging, limited dimensional changes with temperature and humidity, and long service life without the possibility of abrasion or damage. Electrostatic charging is a problem particularly serious for the transparent sheet of clear plastics having a high surface resistivity. A statically charged lubricant sheet unfavorably affects the passage of the tape, increases the torque requirement, and reduces the tape life or frequency of usages. The sheet is charged also when it is stamped out or in transportation, and if some electric charge of the same polarity is built up in the case during the assembling the tape into the case, the combined charge can force the sheet out of place. In the mechanized assembly line, the lubricant sheet off the proper place is incorporated as it is into the assembly, resulting in a defective product. Also, the charge often causes the sheet to shift out of place while the tape is running. Experiments indicated that the percentage of shifting of the sheet out of normal place during assembling work was as high as 58% and that of troubles in running was 35%.
The transparent sheet is commercially available in a variety of kinds. However, from the viewpoints of friction coefficient and static chargeability, none of those are suitable for immediate use, without any additional processing or treatment. For example, polyesters have high surface resistivities of 10.sup.16 .OMEGA..multidot.cm or upwards. In view of this, the present inventors have attempted at decreasing the coefficient of friction by applying a release type silicone coating to a transparent sheet of commerce as the base and also at preventing the electrostatic charging by giving an antistatic coating to the base. Thus, as the clear lubricant sheets designed to solve the both problems of high friction coefficient and static chargeability, varied lubricant sheets have been developed which consist of a transparent base of polyethylene terephthalate, polypropylene, polyethylene, polymethylene terephthalate, polycarbonate, cellophane, or styrol, provided with a release type silicone coating on one side and an antistatic coating on the other, or provided with a release type silicone coating that incorporates an antistatic agent on either side or both sides of the base.
It has, however, been found that even the lubricant sheets having the antistatic coating or silicone coating containing an antistatic agent presents other difficulties. Firstly, in the case of the lubricant sheet having the release type silicone coating on one side and the antistatic coating on the opposite, the two coatings both of which are transparent can hardly be distinguished from each other. The silicone coating must come to the side of the sheet that makes contact with the tape. Should it be erroneously replaced by the antistatically coated side during the course of assembling, the latter coating with a high coefficient of friction would invite a trouble of the running tape, making the lubricant sheet useless as such. Secondly, because the antistatic coating or antistat-containing layer is exposed to direct contact with the tape, the lubricant sheet will lose its antistatic effect as the antistatic agent wears off with about 30 passes of the tape. Moreover, the agent can evaporate completely during a heat-resistance test or on standing. Thirdly, incorporation of an antistatic agent into release type silicone lowers the good surface friction coefficient characteristic the silicone coating originally possesses and, furthermore, the proportion of the agent to be mixed with the silicone is limited because the coating layer is thin .